Preheat your oven to 350° F (180° C). Line your cookie sheet with either parchment paper or a silicone baking mat. Set aside.
In the bowl of a stand mixer or with an electric hand mixer, cream together the butter, brown sugar and granulated sugar for 2 minutes.
Add the egg and vanilla, mixing until thoroughly combined.
Add in the flour, baking soda and salt. Mix until your dough comes together.
Lastly, add in the M&M's. Mix until evenly distributed.
Use a cookie scoop to portion out your dough into your desired size. For small cookies, use 1 1/2 tbsp of dough (#50 scoop). For medium cookies use 3 tbsp of dough and for extra large, bakery sized cookies use 1/3 cup of dough.
Arrange on your prepared pan and bake at 350° F (180° C) until the cookies have flattened, spread and are just starting to turn golden brown on the edges. For small cookies, bake 7-9 minutes. For medium cookies, bake 9-11 minutes. for large cookies, bake 11-13 minutes.
Allow the cookies to cool on the pan. Doing this will help finish cooking the insides of the cookie, so it is okay if the cookie looks slightly under-baked when you pull them out of the oven.
Store in an airtight container on the counter for up to 3 days, or in the freezer for up to 3 months.